In fluid line systems for the transfer of fluids, including pipelines, control lines and the like, control apparatus may be employed to maintain and control system function and operation. An area of particular interest in petroleum transfer systems is apparatus for leak detection, as may be used for example, in underground gasoline lines at service stations, wherein leakage cannot be visually determined. Such apparatus may include differential pressure switching to detect a differential pressure condition between two points in the system. Should a fluid leak develop in the system on one side of the switch, a differential pressure relative to the other side of the switch will result. When a preset differential pressure is reached, the switch is activated, and a visual indicator or other signal is generated. When the differential pressure has subsided to a preselected level, the switch is deactivated.
An example of a typical prior art differential pressure switch is shown in FIG. 1. The switch includes first inlet I.sub.1 extending into first cavity C.sub.1, second inlet I.sub.2 extending into second cavity C.sub.2, diaphragm D separating cavities C.sub.1 and C.sub.2, and trigger T attached to diaphragm D which contacts plunger P of microswitch S. The microswitch is activated by displacement of the diaphragm. Displacement occurs when the pressure in pressure cavity C.sub.1 on one side of the diaphragm is greater than the pressure in pressure cavity C.sub.2 on the opposite side of the diaphragm. When the diaphragm moves the plunger to a preset limit, the microswitch is activated.
Microswitch S has a relatively large physical force vs. displacement hysteresis during activation and deactivation of the electro-mechanical switch components. FIG. 2 illustrates this effect by showing the difference in the diaphragm force-displacement curves representing the switch activation and deactivation modes. The hysteresis occurs due to the natural operational characteristics of the microswitch, which incorporates a set of mechanical contacts that must be moved against an internal spring to achieve positive On-Off motion using a "snap action." This basic characteristic poses a problem when trying to measure extremely small differential pressures, such as those in the range of 1-5 inches of water column (that is, about 0.04-0.19 psi) with a small volumetric displacement. Although pressure sensitivity can be improved somewhat by increasing the size of the switch diaphragm so as to increase the force of the fluid thereon, enlarged diaphragms require a concommitant increase in volume differential for activation and still exhibit the same unacceptably high hysteresis. Such switches are therefore unsuitable for low pressure, low flow systems. Moreover, such switches may be unduly susceptible to caustic materials, such as petroleum, and the electrical sensing elements thereof may present an explosion hazard.
An alternative configuration of the standard differential pressure switch incorporates a snap action Belville spring. This, however, increases the hysteresis of the force vs. displacement of the diaphragm.